In general, an antenna device for a portable terminal refers to a device that allows a user to use mobile communication services through a portable terminal by performing wireless communications with a mobile communication base station.
Mobile communication services provided through a portable terminal at the initial stage were, for example, voice communications and transmission of short messages which were commercialized by simple communications through which data of low capacity were transmitted and received at a low speed. However, in recent years, various types of services such as transmission of videos, real-time searches through the internet, digital multimedia broadcasting (hereinafter, referred to as ‘DMB’) are being provided, so it is required to transmit and receive data of high capacity at a high speed. Thus, in an antenna device for a portable terminal, it is required not only to optimally utilize a bandwidth defined at its operation frequency but also to simultaneously receive various services provided at different frequency bands if necessary. For example, in order to use a mobile communication service and a DMB service through a single portable terminal, it is necessary for the portable terminal to simultaneously receive services provided at different frequency bands.
Due to many efforts for improving the performances of antenna devices according to changes in the mobile communication service environment, there have been many difficulties in realizing antenna devices capable of simultaneously receiving mobile communication services provided at different frequency bands while it becomes possible to transmit data of high capacity through which videos can be watched and an internet service can be used in real time at a high speed.
That is, when an antenna device is manufactured by combining antenna elements operated at different frequency bands, the operation characteristics of the antenna elements are unavoidably distorted due to the interferences between the antenna elements. Thus, there have been many trials and errors in optimizing an antenna device of dual bands.
Moreover, an antenna device has an electrical length which is a half or a quarter of an operation frequency wavelength, whereas since a mobile communication service is provided at a relatively high frequency band (for example, 900 MHz, 1.8 GHz, and 2.1 GHz), the antenna device can be easily miniaturized and can thus be mounted to the interior of a terminal. However, it is unavoidable to increase the electrical length or volume of an antenna device in order to use a service, such as a ground-wave DMB, which is provided at a low frequency band (for example, 200 MHz). Thus, when an antenna device for using a service of low frequency band is embedded in a terminal, the size of the terminal becomes larger and it becomes inconvenient to carry the terminal.
Due to the above problems, while some portable terminals are miniaturized, detachable antenna modules for using ground-wave DMB services are provided in the portable terminals separately. However, in this case, it is inconvenient to carry a detachable antenna module and it is also bothersome for a user to couple an antenna module in order to watch a DMB service.